Brown hills, bad air and bald ski runs may already have made that clear, but numbers made available on Tuesday proved just how dry it’s been.

In San Francisco, which has rainfall records going back to the Gold Rush, last month was the third-driest December since 1849. Only two Decembers were drier, and in those months — in 1876 and 1989 — no rain fell at all.

The story is much the same for the rest of the Bay Area. In San Jose, it was the third-driest December since 1874. In Oakland, it was the fifth-driest since 1970, when consistent records were first kept.

And the forecast calls for continued dry conditions for at least the next 10 days.

“It’s fairly typical to get a dry spell but not to have this prolonged of a dry spell in the middle of winter,” said meteorologist Jan Null, owner of Golden Gate Weather Services in Saratoga. “This is going to be all of December and a good part of January.”

Meanwhile, the state’s first snow survey of 2012 on Tuesday showed that the snow pack in the Sierra Nevada is only 19 percent of normal.

But state water officials said that — for now at least — they aren’t worried because heavy rains last winter left many of the state’s largest reservoirs, including Shasta, Oroville and San Luis, at levels that are higher than historic averages for this time of year.

“Fortunately, we have most of winter ahead of us, and our reservoir storage is good,” said Mark Cowin, director of the state Department of Water Resources.

The culprit for the dry winter is a ridge of high pressure that is pushing the jet stream north. It shows no signs of budging.

The dry, still air led air quality regulators to call 12 Spare the Air alerts since Dec. 1, the most called in the four years of the winter air-quality program. And the winter Spare the Air season is only half over, said Kristine Roselius of the Bay Area Air Quality Management District.

Bay Area water managers appear to have enough water to avoid shortages this summer, but some may nevertheless take steps to conserve water now to avoid tapping into 2013’s reserves.

“Right now we’re in good shape, but we’re in watch mode,” said Charles Hardy, a spokesman for the East Bay Municipal Utility District.

As of Tuesday morning, the 10 reservoirs operated by the Santa Clara Valley Water District were 44 percent full. At this time last year, they were 42 percent full. Both totals reflect a more robust water picture than in recent years, when the level dipped to as low as 13 percent in early January.

“A dry December can be an early warning sign of an upcoming dry year,” said Marty Grimes, a spokesman for the district. “However, we never know what type of water year is coming, and there’s always the potential for a series of dry years.”

Grimes said Silicon Valley is in relatively good shape this year because storms last year boosted water supplies in underground aquifers. That meant the district didn’t have to draw its reservoirs down to low levels.

South Bay reservoirs could be holding even more water today, but district officials had to release some water last year during heavy rains to comply with state seismic safety rules that allow all 10 of the district’s aging reservoirs to be only about three-quarters full.

Meanwhile, businesses that rely on a steady supply of snow are watching warily. At Lake Tahoe, one lakeside restaurant said it was reopening its deck in January, a first for the establishment. The pitch: Come on up, enjoy the sunshine and pray for snow.

Resorts are taking the same approach.

“This is still early season for us,” said Eric Doyne, a spokesman for Ski Lake Tahoe, an association of seven ski areas. “We know the snow is going to come, and when it does, it can come in a big way.”

Paul Rogers has covered a wide range of issues for The Mercury News since 1989, including water, oceans, energy, logging, parks, endangered species, toxics and climate change. He also works as managing editor of the Science team at KQED, the PBS and NPR station in San Francisco, and has taught science writing at UC Berkeley and UC Santa Cruz.